Impedance measurement is a technique presented in literature and already well studied since the sixties. In this document references will be done to patents that deal with similar problems, and then the differences between the present invention and those patents will be highlighted to clarify improvement and adaptation needed to the specific aspects of the current application.
The impedance measurement consists in evaluating the variation of body impedance after injecting an high frequency and low intensity current through two electrodes and measuring the voltage drop caused by such current. Measured impedance depends on the body composition and the flows of fluids across the body itself. Hence the different applications based on this technique: in nutrition science for calculating the body mass index, in cardiology (impedance cardiography) to monitor the cardiac output, in pneumology to check breathing, etc.
The Impedance Pneumography allows to monitor the respiratory activity relying on the thorax impedance variations determined by the air flow through lungs. An Impedancemetry system that uses only two electrodes shows acceptable results only with subjects in rest conditions. In order to improve the performance of the system, it is necessary to use a four wire measurement method, thus making use of four electrodes; the outer ones are exclusively used to inject a high frequency and low intensity current, while the inner ones allow the measurement of voltage variations due to the impedance changes caused by the respiratory activity.
The four electrodes measurement method, thanks to the high input impedance of the measurement amplifier, allows to remove the contribution of the resistance of the cable wire and the contact resistance between the electrodes and the skin, which could significantly alter the impedance we are trying to measure. Furthermore, the use of two electrodes to inject current, which are different from those used to measure the voltage, lets the spatial distribution of the current density between the measuring electrodes be approximately constant.
This allows to reduce the effect of impedance variations in proximity of the “injection” electrodes due to the higher current density in their proximity. Since measured impedance is function of the air volume present in lungs, such method can be calibrated with a spirometer.
The relationship between impedance change (ΔZ) and air volume change (ΔV) is approximately linear under most circumstances.
Coefficient ΔZ/ΔV depends on physical dimensions of the subject and on the positioning of electrodes and is comprised between 0.3 and 1 ohm/liter. Hence the impedance variation corresponding to each respiratory cycle is inferior to 1% of the basic impedance.
In U.S. Pat. No. 3,802,419 in order to measure the voltage determined by the passage of high frequency current injected through a couple of surface electrodes, a standard full wave diode rectifier is used.
U.S. Pat. No. 3,882,851 indicates the use of a synchronous demodulator in the block diagram, the output signal of which is used to control the amplitude of the stimulating current. This is used to guarantee that the variations of the measured voltage lie within an acceptable range, in order to compensate the drifts from which such a solution suffers.